1. Field of the Invention
The present invention relates to an organomineral decontamination gel that can be used for radioactive decontamination of surfaces, in particular metal surfaces.
2. Description of the Background
The decontamination of parts soiled by radioactive elements can be conducted either by mechanical treatment or by chemical treatment.
The methods which use mechanical treatment have the disadvantage of producing a more or less substantial change in the surface of the part, and also of being difficult to implement on parts of complicated shape.
Soaking treatment methods which consist essentially of removing the radioactive elements fixed on the surface of the part with solutions of appropriate active decontaminant agents, in particular Ce(IV) stabilized in a concentrated strong acid medium such as nitric or sulphuric acid, have the disadvantage of leading to the production of considerable volumes of effluent whose subsequent treatment, by concentration in particular, is very costly.
Also, soaking methods using solutions raise some problems for the treatment of large-size parts that are difficult to immerse and to soak entirely in the solution of reagents.
With decontamination solutions, treatment by soaking can only be applied to metal parts of restricted size which can be dismounted, that is to say that these solutions can only be used in practice when radioactive installations are being dismantled.
Also, the on-site decontamination of radioactive installations by spraying aqueous solutions produces large quantities of radioactive effluent and only has limited efficacy on account of the short contact time with the parts.
The idea was therefore put forward to viscose decontamination solutions comprising an active agent with viscosing/gelling agents, in particular with divided solids having a large specific surface area, elementary particles of small size and chemically inert.
Among the solids meeting these requirements, mineral base materials such as aluminas and silicas available on the market, which also offer a large diversity in characteristics such as hydrophilic, hydrophobic properties, pH . . . appear to be the best means for viscosing/gelling these solutions.
Spraying such gels, unlike solutions, can make possible the on-site decontamination of large-size metal surfaces which are not necessarily horizontal but which may also be inclined or even vertical.
Decontamination gels may, therefore, be described as colloid solutions comprising a viscosing agent that is generally mineral such as alumina or silica, and a decontamination agent, for example an acid, a base, an oxidizing agent, a reducing agent or a mixture of the latter, chosen in particular in relation to the type of surface contamination.
Hence an alkaline gel for stainless and ferrite steels will offer degreasing properties for the removal of non-fixed contamination.
An oxidizing gel for stainless steels for the removal of heat or cold fixed contamination. A reducing gel will preferably be used in addition to and alternate with the oxidizing gel to dissolve heat-formed oxides for example in the primary circuit of pressurized water reactors (PWR).
Finally, an acid gel for ferrite steels will remove cold fixed contamination.
The use of gels for the radioactive decontamination of parts is described in particular in document FR-A-2 380 624.
In this document, a decontaminant gel is used that is made up of a colloid solution of an organic or mineral compound to which may be added a decontaminant product such as hydrochloric acid, stannous chloride, sodium oxine and/or fluoride.
Although these gels give satisfactory results, they nevertheless have the disadvantage of only being able to remove encrusted radioactivity to a short depth of the part's surface, for example to a depth of approximately 1 .mu.m.
Document FR-A-2 656 949 describes an oxidizing decontaminant gel which can be used to remove radioactive elements deposited on the part as well as radioactive elements encrusted on its surface.
This decontaminant gel is made up of a colloid solution comprising:
a) 8 to 25% by weight of a mineral gelling agent, preferably silica-based, preferably pyrogenous silica or alumina, PA1 b) 3 to 10 mol/l of an oxidizing agent such as Ce.sup.IV, Co.sup.III or Ag.sup.II having a standard electrode potential E.sub.0 of more than 1400 mV/SHE (standard hydrogen electrode) in a strong acid medium (pH&lt;1) or the reduced form of this oxidizing agent. PA1 a) 20 to 30% by weight of a mineral gelling agent, preferably alumina-based, PA1 b) 0.1 to 14 mol/l of a mineral base, such as NaOH or KOH, and PA1 c) 0.1 to 4.5 mol/l of a reducing agent having a standard electrode potential E.sub.0 of less than -600 mV/SHE in a strong base medium (pH=13) chosen from among borohydrides, sulphites, hydrosulphites, sulphides, hypophosphites, zinc and hydrazine. PA1 a) a viscosing agent PA1 b) an active decontamination agent characterized in that viscosing agent a) comprises the combination of a mineral viscosing agent with an organic viscosing agent (coviscosant) chosen from among hydrosoluble organic polymers and surfactants. PA1 Ce.sup.III /Ce.sup.IV Eo/SHE=1610 mV PA1 Co.sup.II/Co.sup.III Eo/SHE=1820 mV PA1 Ag.sup.I /Ag.sup.III Eo/SHE=1920 mV PA1 0.6 to 1.5 mol/l, preferably 1 mol/l of (NH.sub.4).sub.2 Ce(NO.sub.3).sub.6 or Ce(NO.sub.3).sub.4, PA1 2 to 3 mol/l, preferably 2.88 mol/l of HNO.sub.3, PA1 4 to 6% by weight, preferably 5% by weight of silica, PA1 0.2 to 2% by weight, preferably 1% by weight of a polyoxyethylene ether. PA1 spraying onto the surface to be decontaminated a soda solution for a period of 30 minutes for example, PA1 rinsing with water PA1 applying onto the surface thus treated an oxidizing gel in an acid medium and maintaining it on the surface for a period of 30 minutes to 5 hours, preferably for two hours, PA1 rinsing with water.
In the latter case, the gel also comprises 0.1 to 1 mol/l of a compound d) able to oxidize the reduced form of this oxidizing agent.
In the above-described decontaminant gel, the presence of constituents b) and c) respectively ensures the removal of radioactive deposits formed on the surface of the part and the removal of encrusted radioactivity, through controlled erosion of the surface to be decontaminated.
This oxidizing gel does not, however, have sufficient efficacy vis-a-vis the adherent metal oxide layers deposited on the surface of alloys such as autenite steels, Inconel 600 and Incoloy.
Document FR-A-2 695 839 therefore describes a reducing decontaminant gel which can be used to remove these layers of adherent metal oxide, which comprises :
The application of the gels to the surface, for example the metal surface, to be decontaminated is preferably made by gun spraying, for example under a pressure that may range from 50 to 160 bars and even higher, the gel being shaken before spraying to homogenize the gel. After adequate action time, the gel is rinsed by spraying water, and the effluent generated is treated for example by neutralization, decantation and filtration.
All the gels described above, whether alkaline, acid, reducing or oxidizing, in addition to the advantages already described above such as the possible treatment of parts of complex shape, also have the advantages of easy implementation, low quantity of chemical reagents sprayed per surface unit, therefore a small quantity of effluent produced when rinsing the applied gels, full control over surface contact time and therefore control over erosion during decontamination. In addition, since it is possible to spray the gel from a distance, the doses absorbed by staff in charge of carrying out radioactive cleaning are greatly reduced.
Typical gels of the prior art are marketed by FEVDI under the trade name "FEVDIRAD ".
All the above gels, whether acid, alkaline, oxidizing or reducing also offer good corrosive properties, especially the oxidizing gels,.
Unfortunately, they cannot tolerate high shear speeds needed for spraying, which is the most conventional process for applying these gels.
All these gels comprising a mineral viscosing agent, silica in particular, whether hydrophilic, hydrophobic, basic or acid, have Theological properties that are characteristically thixotropic: their viscosity decreases under shear forces during spraying, followed by restructuration of the gel and surface adhesion when shearing stops. A rheogram showing hysteresis characterizes the response of this type of fluid.
Control over such thixotropy is of fundamental importance to obtain optimal spraying and adhesion of the gel to the surface to be treated. The quick re-setting time of the gels, or their full or partial restructuring, constitutes the essential concept of their spraying.
Restructuration denotes a return to gel state and therefore adhesion to the surface, and a short re-setting time characterizes a gel which swiftly recovers sufficient viscosity after spraying to prevent any dripping.
Regardless of the mineral viscosing agent content of the above-described gels or those currently marketed, re-setting times are too long. For example, for various content levels of Cab-O-Sil M5, which is an acid, hydrophilic pyrogenous silica marketed by DEGUSSA, the re-setting times are always longer than 5 seconds, which is far too excessive.
Return time to sufficient viscosity so that the gel can adhere to the wall may be reduced, it is true, but this requires a substantial increase in the mineral conten.
Viscosity under shaking before spraying is high in this case, and spraying becomes difficult. Also, this higher mineral load generates substantial quantities of effluent on rinsing and solid waste to be treated.
For example, 20 kg of gel, after treatment by filtration of the rinse effluent, currently produce a volume of radioactive waste equivalent to a 200 1 drum.
A need therefore exists to improve the Theological properties of existing gels whose gelling/viscosing agent is solely silica or alumina-based, in particular in order to obtain shorter re-setting times, and therefore to increase gel restructuring capacity while maintaining systems which, when shaken, are sufficiently liquid to allow spraying.